This invention relates generally to die heads for producing meltblown fibers from a thermoplastic polymer and nonwoven webs from such meltblown fibers, and more particularly concerns a meltblown die head in which the die head includes a gas reservoir extending the length and width of the die head and containing a volume of the heated gas which is used for the meltblowing process, for maintaining the temperature of the thermoplastic polymer, and for maintaing a constant temperature along the length of the die head.
Forming nonwoven webs of thermoplastic fibers by meltblowing is well known in the art and described in various patents and publications, including Naval Research Laboratory Report No. 4364, "Manufacture of Super-fine Organic Fibers" by V. A. Wendt, E. L. Boon, and C. D. Fluharty; Naval Research Laboratory Report No. 5265, "An Improved Device for the Formation of Super-fine Thermoplastic Fibers" by K. D. Lawrence, R. T. Lukas, and J. A. Young; U.S. Pat. Nos. 3,849,241 to Buntin, et al.; 3,676,242 issued to Prentice; and 3,981,650 to Page.
In general, meltblowing employs an extruder to force a hot melt of thermoplastic material through a row of fine orifices in a die tip of a die head into converging high velocity streams of heated gas, usually air, arranged on each side of the extrusion orifice. A conventional die head is disclosed in U.S. Pat. No. 3,825,380 to Harding et al.
As the hot melt exits the orifices, it encounters the high velocity heated gas stream, and the stream of thermoplastic material is attenuated by the gas and broken into discrete fibers which are then deposited on a moving collector surface, usually a foraminous belt, to form a web of thermoplastic material.
In accordance with conventional practice, the die head is provided with heaters adjacent the die tip in order to maintain the temperature of the polymer as it is introduced into the orifice of the dip tip through feed channels. For example, McAmish et al. U.S. Pat. No. 4,622,259 discloses a conventional die head in which the hot melt is forced from the extruder into a heater chamber located between the die plates that form the die tip. The hot melt is heated in the die tip by means of auxiliary heating elements embedded in the die tip itself. Such a heating method requires heating elements along the entire length of the die head assembly and sophisticated sensors to insure a constant temperature for the polymer from one end of the die head to the other. In addition, when auxiliary heating elements are employed it is necessary for the die assembly to have a high thermal mass to insure even distribution and maintenance of temperature along the length of the die head. If the temperature is allowed to fluctuate across the length of the die head, the meltblown fibers produced at one position on the die head may have different characteristics than those produced at another position on the die head resulting in a nonwoven meltblown web that is inconsistent in its composition from one edge to the other across its width. A failure of any heating element along the length of the die head will result in unacceptable fiber production adjacent the failed heating element and will require time consuming replacement of the heating element.
Page U.S. Pat. No. 3,970,417 discloses a conventional die head in which a channel for supplying the hot melt to the die tips is machined from the mating halves of the die tip to form a polymer feed channel in the shape of a coat hanger. In order to insure even distribution of the hot melt in such a coat hanger feed channel, electric heaters are provided in the die tip adjacent the feed channel to provide a uniform temperature along the length of the feed channel. Particularly, layers of insulation are provided between the heaters and the outside wall of the die tip which is adjacent to small plenum chambers through which the meltblowing air is supplied. Such insulation is specifically provided to prevent changes in the air temperature from affecting the temperature of the die tip and therefore the hot melt within the coat hanger feed channel.
Likewise, Lohkamp et al. U.S. Pat. No. 3,825,379 shows heaters adjacent the die tip for maintaining the temperature of the hot melt prior to extrusion through the die orifice.
In Buehning U.S. Pat. No. 4,889,476, there is disclosed a die tip having increased die tip thickness to provide a large thermal mass for the die tip to insure maintenance of the desired temperature for the hot melt. The temperature of the massive die tip is maintained by electric cartridge heaters in the die tip. More particularly, the die head is designed so that the attenuating air stream is thermally isolated from the body of the die. Consequently, the attenuating air stream does not affect the creation of and maintenance of the proper temperature of the hot melt during the extrusion of the hot melt.
In summary, conventional meltblown die heads have depended on electric heaters and thermal mass to insure thermal stability. Moreover, in conventional die heads the die tips are isolated from the meltblowing air so the meltblowing air will not affect the temperature of the hot melt in the die tip of the die head.